Preparation of compound semiconductors by fused salt electrolysis



March 3,1970

CURRENT 'JIJ; cU b b ET AL 3,498,894

PREPARATION OF COMPOUND sfimcombucfons .BY FUSED SALT ELEcTRoLYsIs vFiled Jan. 13, 1967 VOLTAGE VOLTAGE-"- mvzmons mm J. cuono mum J. ammoATTORNEY United States Patent US. Cl. 204-61 24 Claims ABSTRACT OF THEDISCLOSURE Techniques are presented for both synthesis and epitaxialcrystal growth of compound semiconductors. Zinc blende structure IIIVand IIVI compounds and IIV I compounds are synthesized and epitaxiallydeposited by fused salt electrolysis. Illustratively, a fused saltsolution for the synthesis of gallium phosphide (GaP) is a singlecrystal layer of GaP was produced epitaxially i on the l11 face of asilicon single crystal using this composition. Electroluminescent diodeshave been produced by selectively doping the GaP layer duringdeposition.

Illustratively, when the electrolytic cell was operated at a temperatureof 800 C. with a cathode current density of 50 milliamperes/cmP, a 100micron thick layer was produced in 20 hours.

INTRODUCTION This invention relates generally to preparation ofcompounds by fused salt electrolysis, and it relates more particularlyto both synthesis and epitaxial growth of compound semiconductorsthereby.

There is a general body of literature relating to synthesis of compoundsby electrolysis from fused salt melts. Illustratively, the literatureincludes the 1958 Ph.D. thesis of P. N. Yocum, The Preparation ofTransition Metal Phosphides by Fused Salt Electrolysis, published byUniversity Microfilms, Ann Arbor, Mich., 1960; and the bookElectrochemistr of Fused Salts by IU K. Delimarskii et al., published byThe Sigma Press, Washington, DC, 1961.

Heretofore, single crystals of compound semiconductors have been grownby chemical vapor deposition, by

recrystallization from a melt, or precipitation from metal solution.Each of these prior techniques for growing or depositing compoundsemiconductor crystals has several disadvantages. Vapor growth ofcompound semiconductors requires extremely careful control of growthparameters and expensive and elaborate apparatus. Further, the

reactant gases are often toxic; and it is sometimes impossible toincorporate desired dopant impurities into the crystal during growth.Recrystallization of compound semiconductors from the melt requiresheating of the compound to its melting point which often causescontamination of the melt from the crucible. Compound semiconductorsusually dissociate measurably at their melting points; and to retaindesired stoichiometry and semiconducting properties, it is usuallynecessary to oppose the dissociation pressure of the compound by atleast an equal partial pressure of the metalloid. As the requiredpressure may be many times atmospheric pressure, crystal growth must becarried out in a heated and pressurized container. Growth of compoundsemiconductor crystals from metal solutions usually yields dendriticcrystals which have limited suitability for device fabrication becauseof their small sizes and irregular shapes.

It is desirable that there be available a technique without thedisadvantages of the noted prior art which produces both synthesis andepitaxial growth of suitably doped single crystals, which employsrelatively simple apparatus and which yields crystals of suitable shapeand crystalline perfection for fabrication of semiconductor devices. Thezinc blende compounds are important semiconductors which have beenextensively investigated in the prior art for semiconductor devices ofwhich electroluminescent diodes and injection lasers are exemplary.Illustratively, electroluminescent p-n diodes of GaP have especiallysuitable characteristics. However, technology has not provided apractical means for production of crystalline layers of GaP which aresuitably doped and sufiiciently pure for the fabrication of desirableelectronic devices.

It is desirable that the following semiconductor compounds be preparedby a technique which does not have the disadvantages of the prior arttechniques:

(a) III-V crystalline semiconductor zinc blende compounds including ametal from the group consisting of Ga, Al, and In with a metalloid fromthe group consisting of P, As, and Sb.

(b) IIVI crystalline semiconductor zinc blende compounds including ametal from the group consisting of Zn, Cd, and Hg with a metalloid fromthe group consisting of S, Se, and Te.

(c) IIV crystalline semiconductor compounds including a metal from thegroup consisting of Zn, Cd, and Hg with a metalloid from the groupconsisting of P, As, and Sb. It has been generally considered in theprior art that: neither the synthesis of semiconductor compounds couldbe achieved by fused salt electrolysis, nor that the desirable qualityand quantity of crystalline deposits by fused salt electrolysis would besuflicient to permit its use for provision of semiconductor compoundsuseful for semiconductor devices and especially for semiconductorcompounds of the zinc blende structure. I

It is an object of this invention to provide preparation of compoundsemiconductors by fused salt electrolysis.

It is another object of this invention to provide both synthesis andepitaxial growth of III-V, IIV, and IIVI semiconductor compounds by theelectrolysis of fused salt solutions.

It is another object of this invention to provide crystalline galliumphosphide of desirable purity and property by fused salt electrolysis.

It is another object of this invention to dope a semiconductorcrystalline deposit during fused salt electrodeposition thereof. I

It is another object of this invention to provide by fused saltelectrolysis an overgrowth of one charge carrier type semiconductor on asubstrate of another charge carrier type semiconductor.

It is another object of this invention to produce semiconductor devicesfrom III-V, IIV, and IIVI compounds by fused salt electrolysis.

It is another object of this invention to provide electroluminescentsolid state devices by fused salt electrolysis.

It is another object of this invention to provide photoluminescentdevices by fused salt electrolysis.

Among the advantages of the practice of this invention are itsoperability at atmospheric pressure and relatively low temperatures andthe controllability of crystal growth by the electrolysis parametersvoltage and current-density. Another advantage obtained by the practiceof this invention for the preparation of zinc blende structuresemiconductor compounds of which the III-V compounds are exemplary, istheir epitaxial crystal growth of suflicient crystalline perfection andpurity such that electroluminescence can be achieved.

Another advantage obtained by the practice of this invention is thepreparation of compound semiconductors without excessive overpressure ofthe metalloid.

Another advantage obtained by the practice of this invention is thepreparation of compound semiconductors with uniform doping of thedeposited crystals during growth.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of a 'preferred embodiment of the invention, as illustratedin the accompanying drawing.

FIG. 1 is a schematic drawing partially in section illustrating anelectrolytic cell useful for obtaining epitaxial growth of asemiconductor compound by fused salt electrolysis according to thepractice of this invention.

FIGS. 2A and 2B present current-voltage curves for the codeposition oftwo exemplary elements useful for an understanding of the theory of thisinvention.

DESCRIPTION OF INVENTION Generally, this invention provides for bothsynthesis and epitaxial growth of semiconductor compounds by fused saltelectrolysis. More specifically, crystalline deposits of zinc blendestructure III-V and II-VI semiconductor compounds and II-V semiconductorcompounds are obtained by fused salt electrolysis from a melt containingions in suificiently electrochemically active form of each of theconstituents of the compound. Illustratively, this invention providescrystalline layers of GaP from fused salt melts having a compositionincluding:

(a) NaPO NaF, and Ga (b) LiCl, KC], Ga O and NaPO or (0) NaCl, KC], Ga Oand NaPO Gallium phosphide (GaP) is a specific example of a III-Vsemiconductor compound which is more generally characterized as oneincluding a metal from the group consisting of Al, Ga, and In with ametalloid from the group consisting of P, As and Sb, i.e., GaP, GaAs,and GaSb; AlP, AlAs, and AlSb; InP, InAs, and InSb, or solid solutionsthereof. In the fused salt solution, com pared to the procedures for GaPin which phosphates are used to produce a phosphide, arsenates are usedto produce arsenides, and antimonates are used to produce antimonides-When the Al or In compounds, analogous to the Ga compounds are desired,those elements in the form of their oxides or halides replace the Ga Oin the solution.

The practice of this invention makes it possible to dope the productcrystals p-ty-pe or n-type during the growth of the crystal byincorporating the appropriate impurities into the fused salt solution.Crystals with grown in p-n junctions are produced by adding sequentiallycompound sources of either p-type or n-type dopants to the solutionduring growth. Alternatively, a layer of p-type material is grown in onefused salt solution, followed by the over-growth of a second layer in aseparate fused salt solution which contains an n-type dopant. Junctionscan also be produced by solution regrowth through the practice of thisinvention on a crystalline layer which may be produced by anothertechnique. Illustratively, a layer of GaP doped with Se, i.e., n-ty'pe,can be electroplated on a layer of GaP doped with Zn, i.e., p-type, andvice versa.

The nature and operation of experimental apparatus and controlparameters therefor will be described in reference to the figures. Theapparatus shown in FIG. 1 consists of an atmosphere chamber of quartzglass heated by electrical windings 12 in insulation 13. Within thechamber 10 there is placed a graphite anode crucible 14 in which thefused salt solution 16 is established. A single crystal siliconsubstrate 18 is appended to rod 20 and is immersed in the fused saltmelt 16. Current is applied between the anode crucible 14 and thesilicon substrate 18 via terminals 22 and 24. Voltage source 26 is 4connected via variable resistance 28 to terminals 22 and 24 to establishterminal 24 positive and terminal 22 negative. The electrolytic cellvoltage is measured between terminals 22 and 24 and cell current ismeasured by ammeter 25 in the series path.

The solution 16 can also be maintained in the molten tate by otherconventional techniques such as radio frequency heating or internalresistance heating. Crucible 14 may be of any material that is notattacked by the molten salt solution such as tantalum or quartz coatedwith pyrolytic graphite. The main function of the atmosphere chamber 10is to protect the crucible 14 and other graphite parts 20 from oxidationby surrounding them with an inert gas atmosphere. For the synthesis of asemiconductor compound, a graphite rod or other inert conductor may beused as a cathode; the silicon or other crystalline substrate isrequired only if epitaxial crystal growth is desired.

The fused salt electrolysis approach of this invention to preparation ofsemiconductor crystals utilizes readily controlled parameters of voltageand current density which illustratively permits regulation of growthrate, dopant concentration, and deposit location.

The general problem of the electrolytic codeposition of two elementswill now be discussed with reference to FIGS. 2A and 2B. In FIG. 2A, thesolid curve, labeled a+b, represents the current-voltage characteristicsof the cathode deposition reactions observed in the electrolysis of asolution containing two ions A+ and B+. This curve is interpreted asbeing essentially a composite of two curves shown in FIG. 2A by thedashed lines labeled a and b, which represent hypotheticalcurrentvoltage characteristics for the independent electrolyticreactions from the same solution A++e A and B++e B. Although onlycomposite curve a+b can be observed experimentally, the curves a and bfor the individual ions can be approximated by observing thecurrentvoltage characteristics for the deposition of A in the absence ofB and for the deposition of B in the absence of A. As shown in FIG. 2A,above some voltage V a current is observed which is due to the dischargeof A ions at the cathode if element A is deposited or evolved there.Between voltages V and V the curves a+b and a coincide, i.e., theobserved current is solely due to the electrolytic deposition of A. Atthe voltage V the discharge of ions B begins, and this reactioncontributes a current which adds to the current contributed by thedischarge of ions A. Thus, the observed current curve a+b, increasesmore rapidly with increasing voltage for voltages greater than V Thisvoltage V at which an abrupt change in the curve a+b is observed is theminimum voltage for codeposition. If the voltage is increased beyond Vthe rate of deposition of the two elements increases, but notproportionately. Where curve a intersects curve b at voltage V thecurrents contributed by the discharge of both A ions and B ions areequal. Since both reactions involve the same number of electrons, therates of deposition are equal.

For the codeposition represented by FIG. 2B, curves a and b neverintersect so there is no voltage at which the discharges A+ and B+contribute equally to the current. The voltage V' at which a breakoccurs in curve a+b also corresponds to the lowest voltage at whichcodeposition can occur. Either type of codeposition illustrated by FIG.2A or FIG. 2B can occur in electrolytic deposition of a compounddependent on such parameters as the nature and concentration of theions, and the composition and temperature of the solution.

The formation of a binary compound AB by an electrolytic reaction is insome respects similar to the codeposion of two elements. Thecurrent-voltage curve for such a reaction may be substantially the sameshape as curve a+b in FIGS. 2A or 2B. The voltage V now represents theminimum voltage necessary for the formation of the compound AB. BetweenV and V in FIG. 2A,

compound AB and excess of element A is deposited.

At V only the compound AB is liberated and beyond V the compound AB andexcess B are liberated.

With regard to the electrolytic formation of III-V compounds, and moreparticularly with regard to the formation of GaP, a current-voltagecharacteristic similar in shape to curve a+b in FIGS. 2A and 2B isobserved. Above some critical voltage, the slope of the currentvoltagecurve increases with an abrupt change in slope. Below this criticalvoltage, only phosphorous is liberated at the cathode; slightly beyondthis critical voltage, GaP formation occurs at a relatively low rate. Atstill higher voltages, GaP formation occurs at a relatively high rate.At the temperature of operation of the electrolytic cell, elementalphosphorous is extremely volatile whereas the dissociation pressure ofGaP is very low. Therefore, phosphorous in excess of that consumed inthe formation of GaP evolves as a gas at the cathode. If the cellvoltage is higher than the voltage at which equiatomic amounts of Ga andP are liberated, the deposit consists of GaP and Ga metal which isnonvolatile at the temperature of the fused salt electrolysis.

The minimum voltage necessary for the deposition of a compound in thepractice of this invention depends on the composition of the fused saltmelt, the temperature of the electrolytic cell operation, and thecomposition and surface condition of the cathode. This minimum voltagecorresponds to the voltage at which an abrupt change in slope in thecurrent-voltage curve occurs so it is readily determined experimentally.In the preparation of compounds by fused salt electrolysis, electrodereactions with certain characteristics are advantageous, i.e., both ofthe constituent elements should deposit at the cathode at approximatelythe same voltage. It is desirable that at least one of the constituentelements be a volatile element and that the compound be nonvolatile. Itis advantageous for high current efiiciencies if the current-voltagedeposition curves for the individual elements intersect so that at somevoltage they deposit at equal rates, i.e., the behavior shown in FIG.2A. The foregoing desirable conditions for the practice of thisinvention for both synthesis and epitaxial crystal growth ofsemiconductor compounds can readily be achieved for:

(a) III-V crystalline semiconductor zinc blende compounds including ametal from the group consisting of Ga, Al, and In with a metalloid fromthe group consisting of P, As, and Sd.

b) II-VI crystalline semiconductor zinc blende compounds including ametal from the group consisting of Zn, Cd, and Hg with a metalloid fromthe group consisting of S, Se, and Te.

(c) II-V crystalline semiconductor compounds including a metal from thegroup consisting of Zn, Cd, and Hg With a metalloid from the groupconsisting of P, As, and Sb.

The metal and metalloid elements of a compound must have depositionpotentials such that they codeposit cathodically from a fused saltsolution or fused salt eutectic solution system.

Fused salt solutions for the synthesis and epitaxial crystal growth ofcompound semiconductors by fused salt electrolysis in the practice ofthis invention consist of three types of constituents: solvents andsolvent modi fiers; sources of metal ions; and sources of metalloidions. Illustratively, a solution for the synthesis of GaP consists of 2moles NaPO /2rn0le NaF, and mole Ga O in which NaPO serves both as thesolvent and as the source of the metalloid P, NaF is a solvent modifierwhich lowers the melting point and viscosity of NaPO and Ga O is thesource of Ga ions. Other solvents which can be used are fused alkalihalides or their mixtures. Other compounds can be used as a source ofphosphorous producing ions, e.g. P phosphates other than NaPO orfluoro-phosphates of the alkali metals. The source of gallium ions canalso be from one of its halides, or from an alkali metal gallate or froma halogallate.

Fused salt mixture for the practice of this invention should bemaintained during the electrolysis at a temperature which is suflicientto melt the solvent and dissolve the metal and metalloid sourcecompounds. The lower temperature limit is determined by the nature ofthe salt mixture, i.e., the solvents melting point. The solvents meltingpoint is modified by the addition of the solute. A further factor whichinfluences the temperature is the solubility of the solute in thesolvent. The upper temperature limitation is determined by thevaporization or the decomposition of any one of the components presentin the fused salt solution and also by the stability of and by thedissociation temperature of the semiconductor compound being deposited.The atmosphere over the solution during the fused salt electrolysisdesirably should not be reactive with it or with the crucible it iscontained in. Inert gases such as argon or helium are suitable as Wellas nitrogen, forming gas or air diluted with nitrogen. As statedhereinbefore the electrolytic cell voltage necessary for the synthesisof GaP, is the voltage at which the two elements Ga and P codeposit atthe cathode. When NaPO is used as solvent and source of phosphorous inthe solution the minimum voltage is approximately 0.4 volt. With thistype solution, the situation represented by FIG. .2B is applicable sothat any voltage up to the deposition potential of sodium may be usedwithout codepositing elemental gallium. The upper limit voltage is thedeposition potential of the other ions in the solution such as thealkali metals. The synthesis of GaP is readily carried out by thepractice of this invention over a wide range of current densities. Forepitaxial growth of a high quality crystal, a low rate of deposition isusually required; and it can be obtained by the use of a low currentdensity.

The solvent in which a fused salt for the practice of this invention isestablished desirably forms an ionicliquid on melting in which compoundsof the desired metals and metalloids are soluble yielding ions which arereduced to the constituent elements at the cathode on electrolysis ofthe solutionso that they are codeposited. In addition, the solventshould not decompose on evaporation at an excessive rate at thetemperature of solution in the electrolytic cell. The solvent should notcontain any ions other than those desired in the product com pound,which have cathodic deposition potentials lower than or equal to thepotential at which the product compound deposits at the cathode of theelectrolytic cell. The solution should desirably be electrolyzcd at acell potential sufficient to codeposit the constituent elements of theresultant compound but low enough that the nonvolatile metal element isnot deposited in excess of the stoichiometry of the desired compound orthat other undesired elements in the solution are depositedcathodically.

When an epitaxial single crystal layer of the product compound isdesired, a crystalline substrate cathode is provided on which epitaxycan occur. The substrate cathode should desirably have a crystalstructure and crystalline orientation such that epitaxy is possible forthe crystal structure and lattice constants of the desired productcompound. Further, the substrate cathode must be an electrical conductorat the temperature of operation of the electrolytic cell and shoulddesirably be non-reactive with the fused salt solution at the cathodicpotential at which the product compound deposits.

For the practice of this invention, the temperature of operation of theelectrolytic cell should desirably be suflicient to melt the solvent andproduce a solution of the compounds which acts as source of the metalions and metalloid ions. The conductivity of the product com- 7; poundat the temperature of' codepositi'on should 'desirably be such that thepotential at the growth surface of the crystal can be maintained at thedeposition potential of the compound without inducing electricalbreakdown in the product crystal.

Another solution suitable for synthesizing GaP in the practice of thisinvention is an alkali-halide solution of NaCl-l-KCl to which is added agallium containing compound and a phosphorous containing compound suchas .Ga O and NaPO respectively, i.e., v

Eutectic solution systems are suitable for the practice of thisinvention. Illustratively, a suitable eutectic solution forthe-preparation of GaP by fused salt electrolysis is KCl+LiCl to whichis added a gallium containing compound such as Ga O and a phosphorouscontaining compound such as NaPO e.g.,

1.2LiCl+ 0.8KC1+0.05Ga O +0.lNaPO In the practice of this invention,doping of crystals during fused salt electrolysis epitaxial growth isreadily achieved by addition of the dopant ion to the fused saltsolution. Illustratively, the addition of ZnO to a melt of 2NaPO+0.5NaF+0.25Ga O provides crystalline GaP which is uniformly dopedp-type. Further, the addition to the fused salt solution of Se ions orTe ions in the form of Na SeO or Na TeO respectively, provides crystalsof GaP that are doped n-type. By alternately using Zn and Se, a p-njunction is readily produced.

EXPERIMENTAL DATA GaP crystals prepared through the practice of thisinvention by a fused salt electrolysis are epitaxial deposits with colorwhich ranges from yellow to amber. X-ray diffraction analysis indicatesa Laue pattern for a single crystal. For other than optimum controlparameters of the melt, there may be obtained a dendritic overgrowth onan original epitaxial layer. By adjustment of the operationalparameters, a continuous epitaxial layer of from microns to 100 micronsis readily obtained.

Exemplary photoluminescence measurements for Zn doped GaP produced bythe practice of this invention at both 77 K. and 42 K. provide thecharacteristic red light of 6840 A. wavelength with relatively highefficiency of energy transformation.

Exemplary electroluminescence measurements of the Zn doped GaP producedby the practice of this invention provides red-orange light withsomewhat less officiency of energy transformation than the notedphotoluminescence measurements.

An optimum temperature range for the metaphosphate fused salt solution2NaPO -l-O.5NaF+0.25 Ga O is 750 C. to 950 C. For the eutectic fusedsalt solution and by the dissociation of the materials being deposited.

The following are illustrative examples of the practice of thisinventionwherein compound semiconductors with the zinc blende structure aresynthesized and/or grown epitaxially. EXAMPLE 1 The GaP was deposited ona single crystal'silicon cathode (chemically polished 111 orientationdisk) at a temperature of 925 C. from a melt consisting of 2NaPO /2NaF,and AGa O at a potential of 1.5 volts and a current density ofapproximately 100 ma./cm. Theproduct formed as a golden 'yellow singlecrystal layer to microns thick which was in turn covered by a thickerpolycrystalline layer. w p

EXAMPLE 2 A mixture of Ga O ,.l IaPO and Na]? was melted in a graphitecrucible and the solution was electroplated using the graphite crucibleas an anode and a graphite rod as the cathode. Apolycrystalline depositof GaP formed the c ode.

. EX E 3 A :mixture 'with the composition l6NaPO 4NaF, and 1621 0 washeated to 850 C. in; a resistance furnace. A potential of -5 volts-wasapplied to thecell with a current of 5 amperes for 1 hour. GaP deposited.as yellow microcrystals at thecathode. The product was identified by X-ray diffraction analysis. The lattice constant was found to be 5.47 A.as compared to 5.45 A. reported in the literature. i 1

EXAMPLE 4 In this example, a sodium metaphosphate electrolyte was usedwith melt composition of: gallium to phosphorous ratio 0.125 to 0.25 andmolality of Ga O solute in 2NaPO /2NaF solvent equals 0.05 to 0.1. Thetemperature range was, 800 C. to 1050 C.; and the electrical conditionswere 0.40 to 6.0 volts, current equals. to 5000 milliamperes withcurrent density range of 12.5 to 625 ma./cm. for an electrode area of 8cm.

The electrodes were polycrystalline graphite; single crystalline 100 and111 Si, single crystalline 1,11 'Ge,.and polycrystalline Ge.

For this example, the optimum results were obtained using a substrate ofsingle crystalline 111 Si with melt temperature of 800 0, cell voltageof 1.5 volts and current density of 50 ma./cm. After a time of 20 hours,a single crystal layer of 100 microns was obtained.

EXAMPLE 5 This example provided GaP dendritic crystals and orientedtriangles as evidence of epitaxial growth. An electrolyte alkali-halidesystem of sodium chloride was used. The composition of the melt was 1molar NaCl, 1 molar KCl, 1 to 0.33 molar ratio Ga/P. Current density was12 mat/cm. for-voltage of 1.2 volts to 1.8 volts.

EXAMPLE 6 The following are exemplary data on the synthesis of All,whichis a III- V, zinc blen'de compound semiconductor. A solution withthe composition 2NaPO 0.5NaF, and 025181 03 was electrolyz'ed at 900 C.with an applied potential of 0.6 volt. A white powder deposited at thecathode. The cathode product was insoluble in water, but dis solvedslowly in" acid yielding phosphine gas and a solution containingaluminum-ions. These reactions confirm the presence of AIF in thecathode product.

EXAMPLE 8 The following are exemplary data on ZnSe which is a II-VI zincblendeflcpmpound semiconductor. A solution with the composition 43 molepercentKCl, 57 mole percent LiCl to which was added a 1:1 molar ratiomixture ,of SeCl; and ZnCl The solution was operated at 500 C. The'cellvoltage was 0.96 viand the current was ma.

9 Epitaxial microcrystals of ZnSe deposited on the single crystal Sicathode.

The following are illustrative examples of procedures wherebysemiconductor doping of the epitaxially grown semiconductor compound bythe practice of this invention was obtained by adding a dopant to themelt.

EXAMPLE 9 The dopant zinc was added to sodium metaphosphate electrolyteas ZnO in a concentration of 2.5 X molar. A deposit of uniformly dopedp-type GaP of 100 microns thickness was obtained after hours on asubstrate of single crystalline 111 Si. The operational parameters weretemperature of 800 C., cell voltage 0.9 volt, and current density of 50ma./cm. The total surface area of the crystal was 8 cm. with areas up to0.5 cm. free of cracks.

EXAMPLE 10 With dopants ZnO and -Na SeO in a sodium metaphosphateelectrolyte, there resulted a thickness of GaP of microns after 20 hourson a substrate of single crystalline 111 Si. The operational parameterswere temperature 805 C., cell voltage of 0.9 volt, and current densityof 62 ma./cm. The resultant layer of GaP was doped by both Zn and Se asrevealed by photoluminescence.

EXAMPLE 11 The following is a description of an exemplary procedure forgrowing p-n junctions in a compound semiconductor in the practice ofthis invention. Two fused salt melts were used. A single crystal ofp-type GaP was grown from a solution containing ZnO as the source of Znbearing ions by electrolysis at a cell voltage of 0.9 volt for 20 hours.After this period of time, the cathode was withdrawn from the p-typedopant solution and transferred to the n-type dopant solution whichcontained Na SeO as a source of selenium bearing ions. A layer of n-typeGaP was deposited over the p-type layer by further electrolysis for 2hours. The p-n junction thus formed was suitable for electroluminescentdiodes and emitted red light on the passage of an electric current.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. Method of synthesizing a semiconductor compound with the zinc blendestructure comprising the steps of:

establishing a fused salt solution having respective ion sources thereinof the members of said compound, said solution having an electrolysiscurrent-voltage characteristic exhibiting a first portion and a secondportion wherein said first portion exhibits a given slope over a lowrange of applied voltage and after a critical voltage said secondportion exhibits a slope greater than the slope of said first portionfor greater applied voltages;

electrolyzing said fused salt solution at an electrolysis voltage withinsaid second portion of said electrolysis current-voltage characteristicof said solution; and

depositing said compound cathodically in accordance with saidelectrolysis current-voltage characteristic of said solution.

2. Method of synthesizing a semiconductor compound with the zinc blendestructure comprising the steps of:

establishing a fused salt solution having first and second ion sourcestherein containing respectively the members of said compound saidsolution having an electroylsis current-voltage characteristicexhibiting a first portion and a second portion wherein said firstportion exhibits a given slope over a low range of applied voltage andafter a critical voltage said second portion exhibits a slope greaterthan the slope of said first portion for greater applied voltages;

electrolyzing said fused salt solution at an electrolysis voltage Withinsaid second portion of said electrolysis current-voltage characteristicof said solution; and

depositing said compound cathodically in accordance with saidelectrolysis current-voltage characteristic of said solution.

3. Method according to claim 2 wherein said semiconductor compound is ofthe III-V class.

4. Method according to claim 2 wherein said semiconductor compound is ofthe II-IV class.

5. Method according to claim 2 wherein said semiconductor compound is ofthe II-V class.

6. Method of synthesizing a solid solution with the zinc blendestructure of a plurality of semiconductor compounds comprising the stepsof:

establishing a fused salt solution having ion sources therein containingthe members of sad compounds; electrolyzing said fused salt solution;and depositing said solid solution cathodically.

7. Method of synthesizing binary compound GaP comprising the steps of:

establishing a fused salt solution having first and second ion sourcestherein containing respectively Ga and P;

electrolyzing said fused salt solution; and

depositing said compound GaP cathodically.

8. Method of depositing a layer of a semiconductor compound including ametal from the group consisting of Ga, Al, and In and a metalloid fromthe group consisting of P, As, and Sb comprising the steps of:

establishing a fused salt solution having a metallic ion from said metalgroup and a metalloid bearing ion from said metalloid group;

electrolyzing said fused salt solution; and

depositing said semiconductor compound cathodically.

9. Method of depositing a layer of a semiconductor compound including ametal from the group consisting of Zn, Cd, and Hg and a metalloid fromthe group consisting of S. Se, and Te comprising the steps of:

establishing a fused salt solution having a metallic ion from said metalgroup and a metalloid bearing ion from said metalloid group;electrolyzing said fused salt solution; and

depositing said semiconductor compound cathodically.

10. Method of depositing a layer of a semiconductor compound including ametal from the group consisting of Zn, Cd, and Hg and a metalloid fromthe group consisting of P, As, and Sb comprising the steps of:

establishing a fused salt solution having a metallic ion from said metalgroup and a metalloid bearing ion from said metalloid group;

electrolyzing said fused salt solution; and

depositing said semiconductor compound cathodically.

11. Method of growing epitaxially a crystalline layer of a semiconductorcompound characterized by the steps of:

establishing a fused salt solution having first and second ion sourcestherein containing respectively the members of said semiconductorcompound; electrolyzing said fused salt solution; and

depositing said semiconductor compound cathodically on a crystallinesubstrate cathode suitable for the epitaxy of said compound crystallinelayer. 12. Method of growing epitaxially a crystalline layer of GaPcomprising the steps of:

establishing a fused salt solution having first and second ion sourcestherein containing respectively Ga and P;

electrolyzing said fused salt solution; and

depositing said crystalline layer of GaP cathodically on a crystallinesubstrate cathode.

11 13. Method of claim 12 wherein said crystalline substrate is singlecrystalline silicon.

14. The method of Claim 12 wherein said fused salt solution consists of2NaPO +0.5NaF+0.25Ga O 15. The method of claim 12 wherein said fusedsalt solution consists of 16. The method of claim 12 wherein said fusedsalt solution consists of 17. Method of growing epitaxially a dopedcrystalline layer of a semiconductor compound comprising the steps of:

establishing a fused salt solution having first and second ion sourcestherein containing respectively the members of said semiconductorcompound;

introducing a dopant ion source into said fused salt solution,

electrolyzing said fused salt solution, and

depositing said doped crystalline layer cathodically.

18. Method of claim 17 wherein said fused salt solution has ion sourcesof Ga and P therein.

19. Method of claim 17 wherein said dopant is Zn and said dopant ionsource is ZnO.

20.. Method of claim 17 wherein said dopant is Se and said dopant ionsource is Na SeO 21. Method of claim 17 wherein said dopant is Te andsaid dopant ion source is Na TeO 22. Method of epitaxially growing a p-njunction in a crystalline region comprising the steps of:

establishing a fused salt solution having first and second ion sourcestherein containing respectively the members of a first semiconductorcompound;

introducing a first dopant ion source containing a first dopant of onecharge carrier type into said fused salt solution;

electrolyzing said fused salt solution;

depositing a first layer of said first semiconductor compoundcathodically;

establishing a second fused salt solution having third and fourth ionsources therein containing respectively the members of a secondsemiconductor compound;

introducing a second dopant ion source containing a second dopant of asecond charge carrier type into said second fused salt solution;

electrolyzing said second fused salt solution; and

depositing a second layer of said second semiconductor compoundcathodically over said first layer of said first semiconductor compoundobtained from said fused salt solution.

23. Method as set forth in claim 22 wherein said first and secondsemiconductor compounds are GaP.

24. Method as set forth in claim 23 wherein said first dopant ion sourceis ZnO, said first dopant is Zn, said sec- 0nd dopant ion source is NaSeO and said second dopant JOHN H. MACK, Primary Examiner D. R.VALENTINE, Assistant Examiner

